You can find out who is right the most by seeing who wins the most prediction contests. Those are the realists. The optimists and pessimists, by contrast, are living in imaginary worlds.
"Will this person treat me fairly/kindly?" Often your optimism/pessimism will be a self fulfilling prophecy.
"Is this rainy day a good thing or a bad thing?" (An example from the article.) It turns out, you get to decide.
The parent comment to my original comment mentioned people who are "fake" optimists. I would describe them as jolly or bubbly often, but ultimately pessimistic. That's why their positivity feels disingenuous. It is a facade or a mask. I appreciate their effort, I am simply drawing a distinction between jolly/grouchy and optimistic/pessimistic.